The permanent magnets in brake magnet systems which have already become known are made of an Alnico alloy. This permanent magnet material has a very small error of temperature, the induction of said magnet material changing only by approximately 0.2% per 10.degree. of temperature change. This relatively small temperature error can be compensated without difficulty by so-called temperature compensation elements. The temperature compensation elements are of a magnetizable material with a low Curie point and temperature related permeability. In most cases, iron alloys with approximately 30% nickel are used. These temperature compensation elements short-circuit a part of the available flux. With a temperature increase, the permeability of the temperature compensation elements is minimized in such a proportion that the induction of the magnet is reduced so that a constant magnetic flux is maintained in the air gap.
The constant magnetic flux in the air gap of a watt-hour meter braking magnet is important, as the braking effort on the rotor disc grows less should a reduction in the available flux occur. This causes the metal braking disc to revolve more quickly, thus showing a too high power consumption.
When using permanent magnet material based on the Alnico alloys with relatively small temperature error, the slight employment of compensating material and the consequent slight cross-leakage of magnetic lines of flux causes only minimum weakening in the effective induction of the permanent magnets. The aforementioned permanent magnet material is, however, subject to crises and is expensive.
One therefore attempted to manufacture brake magnets from permanent magnet materials available in large quantities, e.g., barium, strontium or lead ferrite, with relatively low permeability of nearly one, relatively low remanence and very high coercive field strength. This permanent magnet material produced however a large temperature error, whereby the induction changed by approximately 2% per 10.degree. C. temperature change. As one must expect temperature changes of up to 100.degree. C. with electricity meters, this produces an induction change of up to 20%. Such a high induction change is very hard to compensate. One must also expect a relatively extreme weakening in the available induction in the air gap due to the compensation as previously described. This is even more disadvantageous as the ferrite based permanent magnet materials produce a relatively low remanence of approximately 3,000 to 4,000 G.
A meter brake magnet for a watt-hour meter of a material of this type, with low remanence and very high coercive strength, has already become known through the German Pat. No. 947,543 and the British Pat. No. 1,292,257. In the case of said watt-hour meter brake magnets, two preferably right-angled bodies of the material mentioned are arranged on a vertical and preferably radial plane to the meter braking disc. These magnetic bodies are magnetized in counter direction to the disc of the meter rotor and are covered on both sides by elements of good conductivity, whereby the flux closes via said elements of good conductivity to the opposite elements of good conductivity of counter polarity through the air gap and the disc therein. At the same time, a small plate of a known material with temperature related permeability is positioned on the surface of the magnet arrangement facing the meter rotor, respectively the air gap, between the soft iron bodies covering both sides of the magnetic elements and flush with the pole faces of same, for the purpose of temperature compensation in the meter braking magnet.
In the case of this known embodiment in which the temperature compensation elements lie against the elements of good conductivity opposite the air gap, a change in temperature causes the magnetic density in the air gap to change. Furthermore, it is difficult to fit the temperature compensation element exactly flush, i.e. without air gap, between the elements of good conductivity. Even with the slightest fitting errors, the reproducible values of an exact temperature compensation may not be achieved.
These known brake magnets could not be put into practice as a satisfying temperature compensation in the required temperature range obviously could not be achieved and the manufacturing costs were too high.